The present invention relates to a method for determining an air ratio in a burner for a fuel cell heater as well as a fuel cell heater.
Fuel cells, such as polymer membrane fuel cells for example, are sufficiently known. Fuel cell heaters for decentralized energy supply are fed with natural gas through a gas supply connection, wherein hydrogen is reformed from hydrogenous compounds of the natural gas. In the presence of a catalyst in a reformer, the hydrocarbons (CnHm) of the natural gas undergo endothermic reform by the addition of water vapor, wherein carbon dioxide (CO2) and hydrogen (H2) form.
The reformate also contains residues of carbon monoxide (CO), which are selectively oxidized exothermically in a downstream gas purification by the addition of oxygen (O2). This forms carbon dioxide (CO2) and water (H2O). A gas burner is used for the endothermic steam reformation.
Such reformers are described for example in EP 0 922 666 B1, DE 102 13 326 A1 and EP 1 094 031, the entire contents of all of these listed references are incorporated herein by reference.
A method and a device for operating a gas burner are known from DE 196 18 573 C1, the entire contents of which is incorporated herein by reference, in which the fuel/air ratio, termed a lambda value, is maintained within a pre-defined interval. To regulate the air ratio, an ionization electrode, arranged in the burner's flame range, is connected to an evaluation circuit for a current flowing between the burner and the ionization electrode. The evaluation circuit generates an ionization voltage from an ionization current depending on the combustion which is supplied to a regulator circuit. In regulating the fuel and/or air volume flow in the burner, the ionization voltage is then depending on whether a high-caloric gas or a lower-caloric gas is burned.
An air ratio controller for a burner is known from EP 1 186 831 B1, the entire contents of which is incorporated herein by reference, in which a sensor determines the quality of the combustion. A sensor evaluating device produces a sensor signal supplied to a control unit. The control unit stores characteristic data on the behavior of the actuators. When the ionization signal is not representative of the combustion during the starting operation of the burner or for other reasons, the air/fuel ratio is not regulated but instead controlled. Following a pre-purge period, a notional actuating signal is produced so as to approach the optimally-regulated value as quickly as possible.
The invention is based on the object of providing a method for determining an air ratio in a burner for a fuel cell heater which also allows reliable control of a burner in a fuel cell heater during the starting phase of the system and upon changes in the gas composition.